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Numbers k such that 3*k is not squarefree.
1

%I #10 Oct 08 2023 04:53:02

%S 3,4,6,8,9,12,15,16,18,20,21,24,25,27,28,30,32,33,36,39,40,42,44,45,

%T 48,49,50,51,52,54,56,57,60,63,64,66,68,69,72,75,76,78,80,81,84,87,88,

%U 90,92,93,96,98,99,100,102,104,105,108,111,112,114,116,117,120,121,123,124,125,126,128,129,132,135,136,138

%N Numbers k such that 3*k is not squarefree.

%C The asymptotic density of this sequence is 1 - 9/(2*Pi^2). - _Amiram Eldar_, Oct 08 2023

%t Select[Range[140], !SquareFreeQ[3*#] &] (* _Amiram Eldar_, Oct 08 2023 *)

%o (PARI) isA366288(n) = !issquarefree(3*n);

%Y Complement of A261034.

%Y Union of A013929 and A008585 (after its initial 0).

%Y Positions of zeros in A156277, in A359377, and in A365428.

%Y Cf. A366290 (subsequence).

%K nonn,easy

%O 1,1

%A _Antti Karttunen_, Oct 07 2023